Gyros have been used to measure rotation rates or changes in angular velocity about an axis of rotation. A basic conventional fiber optic gyro (FOG) includes a light source, a beam generating device, and a coil of optical fiber coupled to the beam generating device that encircles an area about a sensing axis. The beam generating device transmits light beams into the coil that propagate in a clockwise (CW) direction and a counter-clockwise (CCW) direction along the core of the optical fiber and both ultimately impinge on a photodetector (e.g., a photodiode electrically coupled to a photodetection system). Many FOGs utilize glass-based optical fibers that conduct light along a solid glass core of the fiber. The two counter-propagating (e.g., CW and CCW) beams experience different optical path lengths while propagating around a rotating closed optical path or loop. For example, rotation about the sensing axis increases the optical path length in one rotational direction and decreases the optical path length in the other rotation direction. The difference in the two optical path lengths introduces a phase shift between the light beams for either rotation direction (i.e., the Sagnac effect), and this difference is proportional to the rotational rate that is normal to the enclosed area.
The phase difference between the counter-propagating light beams in the loop is typically modulated to increase sensitivity to rotation detection using an optical phase modulator coupled with a bias signal generator. The optical phase modulator may be operated in a feedback loop from the photodetection system to provide sufficient negative feedback for canceling the phase shift difference, between the counter-propagating light beams, resulting from a rotation about the sensing axis. In one example, a phase-sensitive detector receives a signal representing the photodetector output current and provides an indication of the relative phase of the light beams impinging on the photodetector. Any significant signal content recovered by the phase-sensitive detector at the modulation frequency is proportional to the rotation rate.
The signal from the phase-sensitive detector may be used to phase shift one light beam relative to the other beam to bring the counter-propagating beams in phase with each other. For example, the signal from the phase-sensitive detector may be supplied to servo electronics having an accumulator, such as an integrator. In response to the phase differences generated during rotation, the servo electronics supplies a signal to phase shift one light beam relative to the other beam. In digital implementations of the feedback loop, an analog-to-digital converter (ADC) converts the analog output of the photodetector to a digital equivalent (e.g., for use by the phase-sensitive detector), and a digital-to-analog converter (DAC) converts the digital signal from the servo electronics to an analog signal for phase shifting via the optical phase modulator.
At very low rotation rates and near zero degrees phase detection, plus or minus some pre-determined value, the gyroscope is generally less sensitive to phase detection. This region is referred to as a “deadband,” and deadband errors may occur for a number of reasons. One source of deadband results from the truncation of digital signals from the servo electronics to lower bit values prior to supplying the digital signal to the DAC. This truncation can also introduce an asymmetry that, in the presence of vibration, could rectify into rate error. Additionally, loop error may take a significant amount of time to converge to the appropriate phase shift, and this time for convergence may be further extended with the truncation of signals supplied to the DAC.
Closed-loop optical sensors, such as the fiber optic gyros discussed above, typically contain an Integrated Optics Circuit (IOC) modulation component that is driven with a 12 to 16 bit truncated signal received from a single high resolution Digital to Analog Converter (DAC). The resolution metrics of these sensors are substantially limited by the data truncation associated with the use of only one high resolution DAC in the IOC drive component of the sensors. In high precision applications, these truncated signals can cause undesirable deadband errors. For example, the least significant bit of a 12-bit resolution DAC scales to a several hundreds of degrees per hour bias error. Even though over a long period of time the closed-loop architecture maintains a bias error which averages to zero, quantization error remains in terms of angle white noise.
One solution utilized to improve bit resolution in closed loop fiber optic gyros is to employ a higher resolution DAC in the IOC drive. Unfortunately, these high resolution converters are expensive and still require truncation well below 23-bit resolution. Another solution utilized to reduce truncation error associated with the use of a single DAC in the IOC drive circuit of fiber optic gyros is to implement a technique that introduces noise and averaging to the signal processing or drive components. Unfortunately, such techniques invariably inject certain unwanted noise into the converter component. This unwanted noise can lead to inconsistent sensor output and decreased system throughput.
Accordingly, it is desirable to provide a closed-loop optical sensor that decreases closed-loop errors. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.